x86: add cpu mtrr init function definitions to mtrr.h

mtrr.h was included everywhere needed.  Fixes the following sparse
warnings.  Also, the return types in the extern definitions were
incorrect.

arch/x86/kernel/cpu/mtrr/amd.c:113:12: warning: symbol 'amd_init_mtrr' was not declared. Should it be static?
arch/x86/kernel/cpu/mtrr/cyrix.c:268:12: warning: symbol 'cyrix_init_mtrr' was not declared. Should it be static?
arch/x86/kernel/cpu/mtrr/centaur.c:218:12: warning: symbol 'centaur_init_mtrr' was not declared. Should it be static?

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>